Field of the Invention
The present invention relates to the production of a foam, for example from molybdenum sulfide (MoS), and more particularly, synthesis of molybdenum sulfide (MoxSy) foam.
Brief Description of the Related Art
There is a great demand for energy worldwide due to increases in population and economic activities in many parts of the world especially in developing countries such as China and India. The energy supply has to keep pace with energy demand. Thus, there are several options for generating energy including fossil-based fuels, solar, nuclear and wind. Many countries have plans to increase the share of renewables in the energy mix and reduce dependence on fossil fuels.
For the latter, fossil fuel-based energy generation relies mainly on natural gas due to its lesser impact on environment and high energy density. Natural gas is produced as an associated gas in oil wells or as sour gas in gas wells, which contains significant amount of hydrogen sulfide (H2S) and carbon dioxide (CO2). When developing sour gas fields, H2S and CO2 have to be removed completely before selling the gas for energy generation. In doing so, the H2S gas is removed by converting it to elemental sulfur through Claus process.
There is a worldwide concern about the increasing environmental effects of oil & gas production. Removal of the sulfur-containing compounds is necessary before utilizing natural gas or any other refinery products.
This large amount of sulfur should bring great motivation and interest for research in this area to come up with different applications that utilize sulfur and hence improve its marketability. One of the forms of utilization is in metal-sulfides, which have a wide range of applications in different industries. Metal sulfides contain at maximum two sulfur atoms per metal and hence have high sulfur content. It follows that with such high content, devising applications for these sulfides will be a suitable way to improve sulfur long-term marketability.
Catalytic chemical processes need materials with high active and accessible surface. MoS2 has been the preferred material option as a catalyst for hydro-desulphurization, which is the catalytic chemical process to remove sulfur from natural gas or other refined chemical products, such as gasoline, jet fuel, kerosene, etc. The molybdenum disulfide MoS2 used to date has a substantially two-dimensional surface, which limits the reactive surface area. One method of increasing the effectiveness of the molybdenum disulphide catalyst would therefore be to create a larger surface area. The challenge is to expand this large surface area into the third dimension.
U.S. Patent Application Publication No. U.S. 2005/0059545 (Alonso et al), granted as U.S. Pat. No. 7,223,713, teaches one method for the synthesis of MoS2 for use as a catalyst with a large surface area. The method involves adding ammonia tetrathiomolybdate salt precursor to a precursor, having an active metal like cobalt. The precursor is decomposed under hydrothermal conditions to form a molybdenum disulfide catalyst in the form of powder, which will also contain carbon.
U.S. Patent Application Publication No. U.S. 2003/0144155 (Tenne et al) also teaches a method of manufacturing a porous matrix of molybdenum disulfide and having nanoparticles of metal chalcogenide inserted into the pores.
Most of the attempts in the past have mainly been approaches to volume applications by using MoS2 powder. One difficulty faced with such application has been the anchoring of the particles of the MoS2 powder into a supporting structure. The supporting structure should not, in itself be an active material, which would otherwise cause inefficiencies in the chemical catalytic process. In the past, additional catalysts and co-catalysts have been used to make MoS2 more active, which is consistent with the industry practice of using chemical treatments instead of making physical improvements.
The literature provides many examples of previous work done with molybdenum sulfide (MoS) relying on conventional synthesis techniques. Other publications describe the improvement of those same techniques with some modifications.
Molybdenum disulfide (MoS2) material has other applications in many industries, which would also benefit from an improved structure. For example, molybdenum disulfide has been used as a lubricant in various applications due to its weak van der Waals bonding between its layers. See, E. Benavente et al, “Intercalation chemistry of molybdenum disulfide,” Coordination Chemistry Reviews, vol. 224, pp. 87-109, 2002. A number of different synthesis methods are known in the literature, which will now be discussed.
Sulfidation of the Oxide.
This method involves solid-gas chemical reaction of the corresponding oxide to produce molybdenum disulfide. It is mostly studied for the field of the hydro-treating processes, in which the molybdenum disulfide is used to enhance the hydro-treating process. The morphology of the produced molybdenum disulfide cannot be easily controlled using this synthesis method. See, P. Afanasiev, “Synthetic approaches to molybdenum sulfide materials,” C. R. Chemie, vol. 11, pp. 159-182, 2008.
Decomposition of Precursors.
Another method for preparing the molybdenum disulfide is through decomposition of a precursor material. The precursor material is a solid/liquid/gas that has the necessary reactants to produce the final product of molybdenum disulfide, where all reaction takes places on a substrate. This method does not involve any external reactant or process steps to get the product. The final product morphology is determined based on the precursor type and the decomposition reaction. See, for example, P. Afanasiev, “Synthetic approaches to molybdenum sulfide materials,” C. R. Chemie, vol. 11, pp. 159-182, 2008. One of the well-known species that acts as a precursor is ammonium tetrathiomolybdate, abbreviated as ATTM or ATM.
Solutions Reaction.
Homogenous reaction that precipitates the molybdenum disulfide as a product is another approach to synthesis the material. This method does not assure the pure products yield of MoS. Other products can be produced from this reaction, like sulfur-rich sulfide, which can be converted to the molybdenum disulfide by thermal decomposition. The slow reaction rate gives better morphology and more control on the MoS2.
The literature has also many examples of how the molybdenum disulfide can be produced directly through a reaction in an aqueous medium. One of the techniques is to use sonochemical synthesis. Nanostructured molybdenum sulfide with high surface area was obtained by this technique. See, M. M. Mdleleni, T. Hyeon, K. S. Suslick, “Sonochemical Synthesis of Nanostructured Molybdenum Sulfide,” Journal of the American chemical Society, vol. 120, pp. 6189-6190. 1998. The nanostructured molybdenum sulfide is prepared by irradiating a slurry solution of molybdenum hexacarbonyl and sulfur along with other chemicals under high intensity ultrasound. Analysis of the produced sample shows larger surface area in comparison to the conventional method of decomposing ammonium tetrathiomolybdate (ATTM) under Helium. Other approaches show the preparations of MoS in aqueous solution by adding surfactants producing high surface area. Metal usually not easily to form aqueous solution and requires H2S or alkali metal sulfides. See, P. Afanasiev, et al, “Surfactant-Assisted Synthesis of Highly Dispersed Molybdenum Sulfide,” Chem. Mater., vol. 11, pp. 3216-3219, 1999.
Surfactant-Assisted Preparation.
This preparation method involves the addition of a surfactant to the preparation technique chosen; it could be either used in either chemical reaction or physical preparations. Surfactants are species that can be attached to the surfaces of the layered material keeping a space between them. In molybdenum sulfide preparations, the surfactant is added to separate the layer of the molybdenum sulfide and control the morphology to fit a desired application. The produced molybdenum sulfide is tested for its mechanical and chemical properties. Other approaches are based on chemical reactions by adding certain types of surfactants to the reactions mixture. See, P. Afanasiev, “Synthetic approaches to molybdenum sulfide materials,” C. R. Chemie, vol. 11, pp. 159-182, 2008.
Intercalation, Exfoliation and Restacking Techniques.
Intercalation can be defined as a process of inserting guest molecules between layered materials in order to ease the separation of the layered materials into single layers. There are two methods for the intercalation step: intercalation of lithium and intercalation of molecular species. The first method is performed by the insertion of alkali metals into the molybdenum sulfide layers such as lithium (Li). It is referred to as a direct intercalation process, which is commonly used rather than other metals. Some work was done to examine intercalation with other alkali metal, see, E. Benavente et al, “Intercalation chemistry of molybdenum disulfide,” Coordination Chemistry Reviews, vol. 224, pp. 87-109, 2002, yet the focus is on Li as it has potential application in high power Li batteries. The process can be described as an ion-electron transfer reaction. Intercalation leads to structural, thermodynamics and reactivity changes in the molybdenum sulfide. Lithium can be intercalated into the molybdenum sulfide layers by two methods: chemical and electrochemical methods. The chemical method is more commonly used, which is carried out by dispersing the molybdenum sulfide in a solution of butyl lithium and organic solvent. See, M. B. Dines, “Lithium intercalation via n-Butylithium of layered transition metal dichalcogenides,” Material Research Bulletin, vol. 10 pp. 287-291, 1975. FIG. 1 shows a schematic of the process of intercalation by lithium. See, E. Benavente et al, “Intercalation chemistry of molybdenum disulfide,” Coordination Chemistry Reviews, vol. 224, pp. 87-109, 2002.
The second method is the intercalation of a molecular species, which is similar to the first method, except that the second method involves the insertion of a compound between the layers. The intercalation step is followed by exfoliation. This step is aimed to remove the molecular species added by a solution that dissolves or react with the molecular species and separate the layers. FIG. 2 adopted from P. Afanasiev, “Synthetic approaches to molybdenum sulfide materials,” C. R. Chemie, vol. 11, pp. 159-182, 2008, shows how this is done. The final step involves adding certain surfactant that fit the material to hold the final structure.
Other than the well-understood approach of Lithium intercalation, different types of guest species have been reported in the literature. Different types of guest species can be intercalated into the molybdenum sulfide, such as polymers, molecular donor, cationic species or organometallic species. A recent work shows that colloidal polymer suspensions of guest species could intercalate the molybdenum sulfide. See, R. Bissessur, P. K. Y. Liu, “Direct insertion of polypyrrole into molybdenum disulfide,” Solid State Ionics, vol. 177, pp. 191-196, 2006.
Another work shows the molybdenum sulfide as thin layers in an electronic application. A thin sheet of molybdenum sulfide has been deposited on insulating substrates. This thin sheet is highly crystalline and has high electron mobility, which matches the properties of the micro-mechanical exfoliated sheets from MoS2 crystals. K. Liu et al, “Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating substrates, Nano letters, vol. 12, pp. 1538-1544. 2012.
An Example of preparation of MoS2/polyvinyl alcohol nanocomposite is known from K. Zhou, S. Jiang, C. Bao, L. Song, B. Wang, G. Tang, et al., “Preparation of poly(vinyl alcohol) nanocomposites with molybdenum disulfide (MoS2): structural characteristics and markedly enhanced properties,” RCS Advances, vol. 2, 2012. The synthesis started with commercial material of MoS2 solvothermal step with butyl lithium (a known chemical for getting Li ions inside the sheets of MoS2) which produces LixMoS2. The exfoliated MoS2 layers were produced from hydrolysis and ultrasonication of LixMoS2 to produce a clean colloidal suspension with MoS2 layers separated. The layers were then added to polyvinyl alcohol polymer solution by solvent blending method, which produces the final mixture that was dried to get the film nanocomposite.
Uses of the molybdenum disulfide catalyst are known, for example, from U.S. Pat. No. 8,673,805 (Anand et al) which teaches the conversion of sugar alcohol to a hydrocarbon.